The present invention relates generally to fluorescent lamps and more particularly to a cathode for a low pressure mercury vapor discharge fluorescent lamp for use with an instant start circuit.
Fluorescent lamps, as known, include a glass tube coated on the inside with phosphor powders which fluoresce when excited by ultraviolet light. The glass tube is filled with rare gases (such as argon, neon, and krypton) and a small amount of mercury, and operates at a relatively low pressure. Electrodes are mounted within the glass tube. The electrodes are coated with an emission mixture, typically comprising carbonates of barium, calcium, and strontium. The carbonates are converted to a ceramic material comprising the corresponding oxides when activated. The emitter material emits electrons during lamp operation. The electrons are accelerated by the voltage across the tube until they collide with mercury atoms, causing the mercury atoms to be ionized and excited. When the mercury atoms return to their normal state, photons corresponding to mercury spectral lines in both the visible and ultraviolet region are generated, thereby exciting the phosphor coating on the inside of the tube to luminance. Fluorescent lamps are becoming widely accepted as plug in replacements for incandescent lamps.
To start a fluorescent lamp, electron emission from the electrodes may be induced in several ways. In a first method, a filament electrode is heated by passing current through it. Such lamps may be referred to as xe2x80x9cpreheatxe2x80x9d lamps. During initial start-up of the preheat lamp, a starter bulb, which acts as a switch, is closed, thus shorting the electrodes together. Current passes through both electrodes, serving to preheat the electrodes. This makes them more susceptible to emission of electrons. After a suitable time period has elapsed, during which the electrodes have warmed up, the starter bulb opens. An electric potential is thus applied between the two electrodes, resulting in electron current between them, with subsequent operation of the lamp. A relatively high voltage is applied initially for starting purposes. Then a lower voltage is used during normal operation. A reactance is placed in series with the lamp to absorb any difference between the applied and operating voltages, in order to prevent damage to the lamp. The reactance, suitable transformers, capacitors, and other required starting and operating components are contained within a device known as a ballast.
In a second method of starting, a high voltage, which is sufficient to start an electric discharge in the lamp, is applied across the lamp without preheating the electrodes. So-called xe2x80x9cinstant startxe2x80x9d circuits which are commonly used today typically employ this method of starting. Such instant start lamps employ ballasts which are much more energy efficient than older style ballasts which heat the electrodes. Since a current does not pass through the electrodes, instant-start lamp electrodes may have only a single terminal, although two terminals may be provided so that the lamp may be used with instant start or other ballasts. An extremely high starting voltage (e.g., up to 500-800 V) is typically applied at high frequency in order to induce current flow without preheating of the electrodes. The high starting voltage is supplied by a special instant-start ballast.
A third type of lamp is known as the xe2x80x9crapid-startxe2x80x9d lamp. A rapid-start ballast contains transformer windings, which continuously provide an appropriate voltage and current for heating of the electrodes. Heating of electrodes permits relatively fast development of an arc from electrode to electrode using only the applied voltage from the secondary windings present in the ballast.
Due to the cost of the components and the sophisticated enclosed fixtures often used, it is desirable to extend the life of fluorescent lamps to reduce replacement costs. Various ways have been developed to increase life. Ballast designs have been improved to obtain a smoother start of the lamp. High frequency rapid and program start ballasts have been developed which first heat the electrode and then either keep the electrode hot during operation (rapid start) or turn the heater current off (program ballasts). Use of high current or high Rh/Rc rapid start ballasts has been found to increase lamp life.
Changing the gas composition has also been found to improve lamp life. For example, increasing the argon pressure above the standard 2.5 torr has been found to decrease diffusion coefficients of the fluorescent lamp cathode species and hence impedes evaporation of the emitter from the coil. Increasing the fill pressure from 2.5 torr to 2.8 torr, for example, has been found to provide a 20% increase in lamp life. However, efficiency of the lamp decreases as the fill pressure increases. The higher pressures also negatively affect the lamp starting voltage.
Another way to increase lamp life is to increase the proportion of heavy gases, such as Kr and Xe, in the fill. This decreases the evaporation of the cathode emitter species and increases lamp life. However, changing the gas composition also changes the wattage of the lamp and starting characteristics.
Despite improvements, lamps using rapid start ballasts typically have longer life than those on instant start ballasts. For example, a typical T8 SP Starcoat(trademark) lamp manufactured by General Electric Company is rated at a life of 20,000 hours on a rapid start ballast and 15,000 hours on a conventional instant start ballast (so called T8 lamps have an internal diameter of 1xe2x80x3. T12 lamps have an internal diameter of 1.5xe2x80x3).
Instant start ballasts are generally easier to manufacture than rapid start ballasts. Thus, several current designs of lamps, such as GE""s F32T8 Ultra Watt Miser and Ultra Watt Miser XL, are designed for use with instant start ballasts. There remains a need for a lamp which operates with an instant start ballast, but which has a lifetime more comparable to or exceeds that of rapid start ballasts.
In an exemplary embodiment of the present invention, a discharge lamp is provided. The lamp includes an envelope. A discharge-sustaining fill is sealed inside the envelope. First and second electrodes provide a discharge. At least the first electrode includes a current carrying wire and a coil including a first coiled structure formed by winding an overwind wire around a first cylindrical member, a second coiled structure formed by winding the first coiled structure around a second cylindrical member, a third coiled structure formed by winding the second coiled structure around a third cylindrical member. The third cylindrical member has a diameter of at least 1.0 mm. An emitter material is deposited on the coil.
In another exemplary embodiment of the present invention, a discharge lamp includes an envelope. A discharge-sustaining fill is sealed inside the envelope. First and second electrodes provide a discharge. At least the first electrode includes a coil including a first coiled structure formed by winding a wire around a first cylindrical member, a second coiled structure formed by winding the first coiled structure around a second cylindrical member. The second coiled structure has coils which are spaced to provide at least 80 turns per inch (TPI). A third coiled structure is formed by winding the second coiled structure around a third cylindrical member. An emitter material is deposited on the coil.
In another exemplary embodiment, a method for forming a coil for a fluorescent lamp is provided. The method includes winding a wire around a first cylindrical member and a current carrying wire to form a first coiled structure. The first coiled structure is wound around a second cylindrical member to form a second coiled structure. The second coiled structure is wound around a third cylindrical member to form a third coiled structure. The third structure has a diameter of at least 1 mm. The third coiled structure is coated with an emitter mix which, when activated, emits electrons when a voltage is applied to the coil.
One advantage of at least one embodiment of the present invention is the provision of an electrode with a longer life, thereby increasing the lifetime of a fluorescent lamp in which it is used.
Another advantage of at least one embodiment of the present invention is the provision of a fluorescent lamp for use with an instant start ballast which has a longer useful life.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.